It is known that in the manufacture of ammonia and urea, large quantities of by-product process condensates are produced. In the manufacture of urea, for example, one mole of by-product water is formed for each mole of urea produced. When this by-product water is considered together with other water used and/or produced in a urea process, the total amount of process condensate and such other water combined can range from 40 to 60 percent by weight of the total weight of the urea being produced. Similarly, in the manufacture of ammonia, the amount of process by-product condensate produced typically can range from 100 to 135 percent by weight of the total weight of the ammonia being produced.
Unfortunately, the by-product process condensates produced in and recovered from such ammonia and urea manufacturing facilities contain contaminants of such a nature that typically they cannot be readily recycled for reuse in the manufacturing processes or discarded to the environment. Generally, contaminants found in the process condensates produced in and recovered from the manufacture of urea can contain not only from 1.0 to 1.5 percent by weight of urea but also from 4 to 8 percent by weight of ammonia and from 2.0 to 5.5 percent by weight of carbon dioxide. The process condensates produced in and recovered from the manufacture of ammonia typically can contain from 0.05 to 0.1 percent by weight of ammonia, from 0.2 to 0.3 percent by weight of carbon dioxide and from 0.05 to 0.2 percent by weight of methanol and, in addition thereto traces of higher alcohols, amines and other hydrocarbon components.
To render the above by-product process condensates useful for recycle in the manufacturing process or readily disposable to the environment, it has been common practice to subject these process condensates to some form of treatment to remove as much of the aforementioned contaminants as possible. Thus, for example, U.S. Pat. No. 4,552,979 describes a continuous process for the treatment of urea plant process condensate containing urea values utilizing both steam and a hydrocarbon fuel gas to hydrolyze the urea values and to strip ammonia and carbon dioxide from the hydrolyzed condensate. The water present in the condensate is converted into steam for use in the process. As an end-product of the process, a gaseous stream comprised of steam, hydrocarbon fuel gas, ammonia and carbon dioxide is recovered which, if the urea plant contains as an integral part thereof an ammonia plant reformer, can be used as a feedstock to such reformer. U.S. Pat. No. 4,341,640 describes an improvement in the treatment of urea plant process condensates requiring the use of a vessel having at least one treating cell comprising an inner cylinder located within the vessel having inside and outside surfaces, and one or more stripping trays connected to the inner surface and a liquid holding zone located between the outer surface of the cylinder and the vessel. This zone is maintained substantially separate from the vapor streams passing through the vessel.
Processes for the treatment of by-product process condensates produced during ammonia manufacture are exemplified by U.S. Pat. No. 3,970,739. This patent describes a process for concurrently treating process waste waters and flue gases from ammonia synthesis process plants. The process involves stripping ammonia and organic materials, as gases, from the process waste water and decomposing the methanol contained in the stripped gas over a first catalyst suited for decomposing methanol to produce a gas containing carbon dioxide and water or carbon dioxide, water and hydrogen. The resulting gas, further containing undecomposed ammonia is mixed with the flue gases from the ammonia plant and the gaseous mixture brought into contact with a second catalyst capable of reducing the nitrogen oxides contained in said gaseous mixture to nitrogen and water. Finally the resulting gaseous mixture is contacted with a third and final catalyst capable of decomposing any unreacted ammonia by oxidation into nitrogen and water.
Exemplary of processes for the removal of urea, ammonia and carbon dioxide from process condensates derived from a coupled ammonia and urea synthesis process is U.S. Pat. No. 4,410,503. This patent describes an improved process where the process condensate resulting from the urea synthesis first is treated in a desorption or stripping column to remove as much of the ammonia and carbon dioxide contained therein as possible and to render said condensate poor with respect to ammonia. Thereafter the ammonia-poor process condensate is introduced into a separate reaction column along with process condensate resulting from the ammonia synthesis and the combined condensates treated with steam. A gas mixture rich in ammonia, carbon dioxide and water vapor is removed from the top of the reaction column and an aqueous liquid poor in said urea, ammonia and carbon dioxide is removed from the bottom of the reaction column. The disadvantage of this process is the need to first treat the process condensate from the urea synthesis prior to combining said condensate, now poor with respect to ammonia, with the process condensate from the ammonia synthesis within the separate reaction column. The need for this separate, first treatment step results in a significant increase in capital costs for equipment required to carry out this first treatment step.
An objective of the present invention is to provide an improved process for treating the by-product process condensates produced and recovered from facilities wherein both ammonia and urea are manufactured. It is a further objective to provide an improved process for the simultaneous treatment of such by-product process condensates in a single treatment vessel thereby reducing the additional capital costs associated with the additional equipment required for the process described in U.S. Pat. No. 4,410,503.